Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game. Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles. ...

Material prepared in connection with a game may be subject to copyright if it contains a sufficient amount of literary or pictorial expression. ...

If your game includes any written element, such as instructions or directions, the Copyright Office recommends that you apply to register it as a literary work. Doing so will allow you to register all copyrightable parts of the game, including any pictorial elements. ...

So if I want to publish a game, I need to make sure that I publish the computer game version first so that I can retain copyright on any derivative works, and be able to create the board or card game later. If I publish the board or card game first, I give up any hope of protecting any future derivative works like a computer game.